The present invention relates to constant-current generating circuits and, more particularly, to a constant-current generating circuit which is independent of the supply voltage and is adapted for use in linear integrated circuits with a low supply voltage.
As is well known, a constant current source can be produced simply by applying a constant voltage to a sufficiently high resistance.
In integrated circuits where one tends to minimize the values of the resistances, the constant-current generators are produced with circuits which comprise, apart from one or more limited-value resistances, active elements such as transistors.
This type of constant-current generating circuit known in the prior art uses a first NPN transistor which is inserted between the two poles of a supply voltage generator in a common emitter configuration.
The transistor base is connected to the positive pole through a resistor and to the negative pole by means of a Zener diode which limits the base potential with respect to the negative pole. The transistor emitter is connected to the negative pole through a suitable emitter resistor, and the collector is connected to the positive pole by means of a diode which, with a second transistor included in the circuit, constitutes a current mirror in whose output branch the collector current of the first transistor is mirrored.
Since the Zener diode limits the base potential of the first transistor and since the base-emitter voltage thereof can be considered constant, a constant voltage is applied to the emitter resistor which produces a constant emitter current and, thus, a constant collector current.
The constant collector current is mirrored in the output branch of the current mirror to which a user circuit can be connected.
Since the internal resistance of the Zener diode is very small, i.e.--negligible, compared to the resistance through which the base of the first transistor is connected to the positive pole of the supply voltage generator, variations of the supply voltage, if any, will not cause appreciable variations in the voltage between the base of the first transistor and the negative pole thanks to the voltage divider formed by the resistor and the Zener diode. Therefore, in a first approximation, the collector current of the first transistor can be considered constant and independent of the supply voltage.
In reality, since the voltage across the emitter resistor is constant, the variations of the supply voltage cause corresponding variations of the collector-emitter voltage of the first transistor and, hence, variations of the collector current as a result of the Early effect.
Therefore, the circuit described above can be used as a constant-current generator only if the supply voltage is only subject to small variations.
A well-known technique which enables one to obtain a constant-current generator that can be integrated monolithically without the use of constant-voltage reference points consists in appropriately coupling two circuits that have a current-mirror structure.
A constant-current generating circuit thus obtained, as known to those skilled in the art, comprises a first current-mirror circuit which includes first and second NPN transistors whose bases are tied together and have emitters that have different emitter areas.
One of the transistors has its base and collector connected as a diode and one resistor is connected to the emitter of the transistor having a larger emitter area; such a resistor and the ratio between the emitter areas of the two transistors are designed such that currents of equal magnitude flow in the collectors of the two transistors.
The circuit comprises a second current-mirror circuit formed by two PNP transistors, one of which has its base and collector connected as a diode, the bases of both transistors being tied together and their emitters having equal areas.
The circuit also comprises one or more output transistors which are suitably coupled, for example, according to a current-mirror structure, to the first or second current-mirror circuit included therein.
The first and second current mirrors, which are inserted between the two poles of a supply voltage generator, can have the collectors of the two PNP transistors directly connected to the collectors of the two NPN transistors according to a simple-ring structure, or coupled by means of cascaded transistors according to more complex structures known in the prior art so as to obtain a higher degree of precision and stability, although it requires a higher minimum supply voltage due to the greater voltage losses.
The prior art described above and a constant-current generating circuit obtained thereby are indicated in an article by Th. J. van Kessel and R. J. van de Plassche entitled "Integrated linear basic circuits" published in "Philips Technical Review" (volume 32, 1971, No. 1, pp. 1-12 with particular reference to FIG. 10 at page 7).
However, it should be noted that even this type of current-generating circuit, due to the Early effect, supplies a constant current which is independent of the supply voltage only within a limited range of possible variations thereof, although much broader than that allowed for the circuit in relation to the supply voltage described previously.